Active servo tracking systems are used in numerous military, industrial, and medical applications. In operation, active servo tracking systems utilize information about a target's motion to correct the physical position of an object to be stabilized in a target frame of reference. The information about the target's motion may be obtained by numerous techniques such as direct position measurements, position correlation, and velocity sensing.
Direct position measurement techniques for obtaining information about a target's motion typically utilize position sensitive detectors, such as quadrant detectors, to detect a "hot spot" associated with the target. Position correlation techniques for obtaining information about a target's motion compare previously stored images of the target to the current image at predetermined time intervals. The resulting image overlap or correlation function is utilized to determine the target displacement.
Velocity sensing techniques for obtaining information about a target's motion typically utilize a signal proportional to the rate of displacement in the frequency domain. The signal is then integrated to give target position information. There are numerous velocity sensing techniques known in the art such as coherent laser-based Doppler and speckle methods.
U.S. Pat. No. 4,856,891 describes an eye fundus tracking system that utilizes active servo tracking and correlation. The system includes a laser source that projects a tracking strip of coherent light on the fundus and optics for producing an image of reflected light from the tracking strip onto a detecting element. The system also includes a means for scanning the intensity profile of the image strip and electronics for analyzing the scanned intensity profile and for providing correction signals which direct the optical path of both the tracking laser beam and a diagnostic laser beam to a fixed position on the fundus. The system, however, is relatively complex to implement.
Numerous applications, such as ophthalmologic and other micro-surgical procedures, require high-speed positioning with accuracy in the cellular dimension range. In addition, it is desirable for such tracking systems to utilize low-power incoherent tracking beams.